Interpretive Summary: Accurate identification of the species that cause invasive fungal infections is critical to facilitate communication of research within the public health community and to advance our understanding of their environmental reservoir. During the past two decades, Fusarium species have emerged as one of the most important groups of opportunistic human pathogens. Members of the Fusarium solani species complex [FSSC] account for approximately two-thirds of all Fusarium infections of humans and other animals. In addition, members of the FSSC also include many economically devastating plant pathogens. The present study of the FSSC was initiated to determine the utility of DNA data to differentiate human pathogenic isolates within the FSSC. Experiments were also conducted to determine whether ten different antifungal drugs were able to inhibit the growth of twenty genetically diverse members of the FSSC. The DNA typing scheme employed was able to differentiate 21 genetically distinct human pathogenic species within the FSSC. Results of the antifungal susceptibility experiments demonstrated that members of the FSSC are broadly resistant to all antifungal drugs tested. Results of the present study will be of interest to and benefit medical personnel concerned with clinical microbiology, infectious diseases, and antifungal susceptibility.

Technical Abstract:
Members of the species-rich Fusarium solani species complex (FSSC) are responsible for approximately two-thirds all fusarioses of humans and other animals. In addition, many economically important phytopathogenic species are nested within this complex. Due to their increasing clinical relevance and because most of the human pathogenic and plant pathogenic FSSC lack Latin binomials, we have extended the multilocus haplotype nomenclatural system introduced in a previous study (Chang et al., JAMA 296:953-963, 2006) to all 35 species within the medically important FSSC clade 3 to facilitate global epidemiological studies. The typing scheme is based on polymorphisms in portions of the following three genes: the internal transcribed spacer (ITS) region plus domains D1+D2 of the nuclear large subunit (LSU) ribosomal RNA, translation elongation factor (EF-1alpha), and the second largest subunit of RNA polymerase II (RPB2). Of the 252 isolates subjected to multilocus DNA sequence typing, 192 sequence types (STs) were differentiated, and these were distributed among three strongly supported clades designated 1, 2 and 3. All of the mycoses-associated isolates were restricted to FSSC clade 3, and these represent at least 21 phylogenetically distinct species. Analyses of the combined DNA sequence data, using 3 separate phylogenetic methods, yielded the most robust hypothesis of evolutionary relationships and genetic diversity within the FSSC to date. The in vitro activity of 10 antifungals tested against 20 isolates representing 18 species that span the breadth of the FSSC phylogeny show that members of this complex are broadly resistant to these drugs.